( information if such information exists. If a message is waiting, it is sent to the computer in response to the polling sequence by means of a Read Modified command operation generated in the terminal. The Read Mod-ified command is not sent from the computer. If neither status/sense information nor a message is waiting, a "no message to send" indicator is sent.
A read-type operation can also be called for by the host computer. In such case, it follows a selection addressing sequence either as the first/only command or as a command chained from another command. However, a read-type command can never be chained from a command which starts a printout operation. Note that the chaining of read-type commands is only permitted after a selection addressing sequence and not after a polling sequence. The communication procedures are described in greater detail in the sections on Remote Operation.
Buffer Start and Stop Addresses
The start position is the buffer address at which a Read Buffer command commences reading and at which a Read Modified command commences scanning for data to send to the host computer. A read-type operation commences at buffer address 0 in the following situations
• When the command is unchained, i.e. when the command follows immediately after a selection addressing sequence
• When the command is chained to a Sense, Select, No Operation or Copy command
• When the command (chained or unchained) is sent to an unformatted screen
If the type command is chained to a Write, Erase/Write or a read-type command, the read-read-type operation always starts at the current buffer address.
A Read Buffer command always reads to the last position in the buffer area, and this also applies to a Read Modified command when the buffer is unformatted, i.e. does not contain any attribute characters. When a Read IVfodified command is issued to a formatted buffer, scanning/reading con-tinues to the last position in the last field. If the first position in the first line does not contain an attribute character, it means that the last field tinues into the first line (wraparound) and scanning/reading can thus con-tinue to the next attribute character.
After a read-type operation is concluded, the buffer address points to position 0, i.e. the first position on the first line unless wraparound took place in connection with a read modified operation. If wraparound took place, the buffer address points to the position of the next attribute character.
4 Commands and Orders [~l
Attention Identification
Most replies that follow a read-type command or a poll include a text part introduced by an AID byte (Attention Identification). The AID byte in-forms the host computer as to which type of activity caused the read-type operation. See Table 2. An AID code other than (60)16 or (E8)16 indicates that the operator at the selected terminal has carried out an operation which requires collaboration with the computer. Examples of such opera-tions include
e Depressing an ENTER, USM, CLEAR, SYREQ, program function or program access key
e Insertion or removal of a card in the magnetic identification device e Using the selector pen or CU SE key to select a selector-pen-detectable
field that initiates transmission
)
)
[~l Commands and Orders 5
Table 2. Attention Identification (AID) Codes
Operator AID codes Command Notes
action EBCDlC16 operation
ENTER key and 70 Read Modified
Selector pen attention (&)
PF 1 key F1
PF14key C2 SBA order (only for
for-PF 15 key C3 matted screen), the field
Card inserted 70 identification code. When
Card removed 60 Read Modified the card is removed only the AID code is transmitted.
The message comprises
Selector pen atten- only the AID code
tion (space, nUll) 7E Read Modified and the field addresses.
No data is transmitted.
PA 1 key 6C Short Read AID code is transmitted only when a Read Buffer command is received from the computer. It is not transmitted for a Read Modified command or polling.
No action at Read Buffer Codes are transmitted
display unit 60 or when a read-type
opera-Read Modified tion is called for by the computer (not polling)
No action at Read Buffer and the terminal has
printer unit E8 or not received a
trans-Read Modified mission request from the operator
6 Commands and Orders I~
Read Buffer Command
The computer can issue a Read Buffer command regardless of operator activity. This command is used primarily for testing purposes and it causes all data (including nulls) from the buffer position at which reading starts to the last buffer position inclusive to be transmitted from the addressed terminal to the computer. Fig. 1 illustrates the format of a response to a Read Buffer command.
Link header
AID Attention Identification
TEXT -First data field, including nulls
Second data field, including nulls Only for formatted screen
Link trailer
Only for formatted screen
Fig. 1. Message from the terminal issued in connection with a Read Buffer command
Read Modified Command
A Read Modified command normally initiates a read modified operation but, depending upon operator actions at the terminal, a short read or a test request read operation can also be initiated. The operations caused by specific operator actions are presented in Table 2 which shows the AID byte codes. In remote configurations, Read Modified commands are not normally sent from the computer due to the fact that the polling sequence initiates a Read Modified command in the terminal if a transmission request is present [AID =l= (60)16 or (E8h6] and no status message is waiting.
For an ENTER key attention, programfunction key attention, selector pen attention (&) or a Read Modified command obtained from the computer, all data fields that have been modified by the keyboard, the selector pen or by a previous Write command are transmitted to the computer. Insignific-ant information is suppressed, i.e. the null characters included in the fields are not transmitted. When a data field is modified by the operator, the Modified Data Tag (MDT) bit is set in the attribute character for the modified data field. When a read modified operation is then carried out, the MDT bit in each attribute character is examined and the data fields for which MDT bits are set are transmitted to the computer. In the message to the computer, each modified data field is preceded by a terminal-generated Set Buffer Address order (SBA), followed by the 2-byte buffer address of the first character position in the field (attribute character address +
O.
See Fig. 2.
)
[~l
(
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Commands and Orders
Link h.eader
AID Attention Identification
} Cursor address, see Appendix 3 SBA Set Buffer Address order
} Attribute character address + 1 TEXT Alphanumeric data
Link trailer
First modified data field*
(nulls suppressed)
Second modified data field (nulls suppressed)
7
Remaining modified data fields (nulls suppressed)
* Note: The SBA order and attribute character address + 1 are not obtained for an unformatted screen. Instead, all text in the buffer (nulls suppressed) follows the cursor address.
Fig. 2. Message from the terminal issued in connection with an ENTER key attention, program function key attention, selector-pen attention (&) or a Read Modified command
For a magnetic identification device attention, a read modified operation is initiated which, when the identification card is inserted, transmits AID and the identification code that was read. When the identification card is removed, only AID is transmitted to the computer. Other data in the terminal buffer is not affected by this read modified operation. Fig. 3 illustrates the format of the message.
Link header
AID Attention Identification
t
Cursor addressJ-~---A
~~~i~~~a~a~~~
___________J
Only in IMS version TEXT Identification code, 0-37 bytesLink trailer
Fig. 3. Messagefrom the terminal issued in connection with a magnetic ident(fication device attention
8 Commands and Orders (~)
For a selector-pen attention, resulting from operation of the selector pen in a field that starts with a space or null character, a read modified operation is initiated which transmits only information indicating which fields were modified by the operator via the keyboard or selector pen. For each selector-pen-detectable field in which the MDT bit is set, a Set Buffer Address order is transmitted as well as the buffer address of the first character position in the data field (attribute character address
+
1). See Fig. 4. No alphanumeric data is transmitted from the modified selector-pen-detectable fields. If any characters have been entered from the keyboard into non-selector-pen-detectable data fields, the addresses of these fields are sent in the same way.Link header
AID Attention Identification b1
Modified data field (if any) address is included
Fig. 4. Message from the terminal issued in connection with a selector pen attention (space or null)
For aprogram access key, USM key or CLEAR key attention, a short read operation is initiated. This means that only AID is transmitted to the computer. In remote configurations, this is, of course, supplemented with data link control characters etc (STX, CD, DV and ETX, for BSC).
For a test request attention, a test request read operation is initiated. This operation, just as a read modified operation, transmits to the computer all data fields that have been modified (MDT = 1) by the keyboard or selector pen. However, in this case, the message is introduced by a terminal-gener-ated Test Request Read heading. See Fig 5. The test request read opera-tion can only be used for terminals that do not use the SN A protocol.
)
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~l Commands and Orders
Link header
---"1'
Start of Heading character
-STX Start of Text character
SBA Set Buffer Address order } Attribute character address + 1 TEXT Alphanumeric data
Link trailer
Test Request Read heading
First modified data field*
(nulls suppressed)
Second modified data field (nulls suppressed)
9
Remaining modified data fields (nulls suppressed)
* Note: For an unformatted screen the SBA order and attribute character address + 1 are not obtained. Instead, all text in the buffer (nulls suppressed) follows STX.
Fig. 5. Message from the terminal issued in connection with a test request attention